With technological advancement and demand for mobile instruments, demand for secondary batteries as an energy source is rapidly increasing. Among such secondary batteries, a lithium secondary battery having high energy density and working potential, a long life cycle, and reduced self-discharge is widely used in the related art.
As to cathode active materials for lithium secondary batteries, lithium containing cobalt oxide (LiCoO2) is widely used. Additionally, lithium containing manganese oxides such as LiMnO2 with a lamellar crystal structure, LiMn2O4 with a spinel crystal structure, etc., and lithium containing nickel oxide (LiNiO2) have also been proposed.
Among such cathode active materials, although LiCoO2 with excellent physical properties such as cycle properties is widely used, this material encounters disadvantages including, for example, low safety, high cost due to scarcity of cobalt as a natural resource, limitation in large-scale use as a power source in electric vehicle applications, and the like.
Lithium manganese oxides such as LiMnO2, LiMn2O4, etc., comprise manganese, which is abundant and environmentally beneficial so as to replace LiCoO2, thus attracting considerable attention. However, such lithium manganese oxide has drawbacks such as a low charge capacity and poor cycle properties.
Meanwhile, lithium nickel based oxide such as LiNiO2 has economic merits, compared to the cobalt oxide and, when charged at 4.3V, exhibits high discharge capacity. A reverse capacity of the doped LiNiO2 is about 200 mAh/g which is more than that of LiCoO2 (about 165 mAh/g). Accordingly, even though average discharge potential and volumetric density are somewhat small, a commercially available battery containing a cathode active material shows improved energy density. Under such circumstances, in order to develop a high capacity battery, studies and investigations into nickel based cathode active materials are actively being conducted. However, practical application of LiNiO2 cathode active materials is substantially restricted owing to the following problems
First, LiNiO2 oxide exhibits rapid phase transition in a crystal structure due to change of volume involved in a charge-discharge cycle, in turn causing particle fracture and generating pores in a grain boundary. Therefore, absorption and discharge of lithium ions are prevented and polarization resistance is increased, thus deteriorating charge-discharge performance. In order to solve these problems, according to a conventional process, Li source is excessively used and reacts in an oxygen atmosphere to produce LiNiO2 oxide. The produced cathode active material has drawbacks in that a structure is expanded and unstable due to atomic repulsion of oxygen atoms during charge of a battery and cycle properties are seriously deteriorated by repeated charge-discharge.
Second, LiNiO2 encounters a problem of excessive gas generation during storage or charge-discharge cycle. This is because heat treatment is performed while excessively adding Li source to form an excellent crystal structure during production of LiNiO2, and therefore, a water-soluble base such as Li2CO3, LiOH, etc. as a reaction residue remains between primary particles and is degraded or reacts with an electrolyte, in turn generating CO2 gas during charge. Furthermore, since a LiNiO2 particle substantially has a secondary particle structure formed by aggregation of primary particles, an area in contact with the electrolyte is increased and the foregoing problem becomes more serious, thus causing swelling of the battery and decreasing high temperature stability.
Third, when LiNiO2 is exposed to air and/or moisture, chemical-resistance is drastically decreased at a surface of the oxide and, due to high pH, an NMP-PVDF slurry begins to be polymerized, in turn causing gellation thereof. The foregoing characteristics may cause serious processing problems in the manufacture of batteries.
Fourth, high quality LiNiO2 cannot be prepared by simple solid-phase reaction, unlike the LiCoO2 production method. Any LiNiMO2 cathode active material comprising Co as a necessary dopant, and other dopants such as Mn, Al, etc. is substantially produced by reacting a lithium material such as LiOH.H2O with a composite transition metal hydroxide under an oxygen atmosphere or a synthetic gas atmosphere (that is, a CO2-free atmosphere), thus requiring high production costs. If any additional process such as washing or coating is conducted in order to remove impurities during production of LiNiO2, production costs are duly increased. Accordingly, conventional technologies have focused in general on improvement of the LiNiO2 production process as well as characteristics of LiNiO2 cathode active material.
A lithium transition metal oxide wherein nickel is partially substituted with other transition metals such as manganese, cobalt, etc. has been proposed. This oxide is a metal-substituted nickel based lithium transition metal oxide with excellent cycle properties and capacity. However, in the case of using the oxide for a long time, cycle properties are drastically deteriorated and other problems such as swelling caused by gas generation in a battery, reduced chemical stability, and so forth, were not sufficiently overcome.
The cause of the foregoing facts is believed to be that: the nickel based lithium transition metal oxide is in a secondary particle form obtained by aggregation of small primary particles; therefore, lithium ions are transported toward a surface of an active material and react with moisture or CO2 in the air to generate impurities such as Li2CO3, LiOH, etc.; impurities generated by residues remained after production of nickel based lithium transition metal oxide may decrease cell capacity; or the impurities are decomposed inside the battery to generate gas, in turn causing swelling of the battery.
Accordingly, there is still a requirement for development of novel techniques to solve high temperature stability problems due to impurities while utilizing a lithium nickel based cathode active material suitable for increasing capacity of a battery.